Hydraulic devices such as reciprocating pumps, compressors or hydraulic drives employing a piston which has a reciprocating movement within a cylinder bore are well known in the art and have been long used for handling different fluids, either liquids or gases. Such a reciprocating pump draws the fluid to be pumped into the cylinder through the pump inlet, during the intake stroke, when the piston moves in one direction, and compresses the fluid within the cylinder when the piston moves in the opposite direction. The pressurized fluid is then discharged via the pump outlet.
Some pumps deliver fluid at high pressures, for example over 4,000 psi and some handle fluids at low temperatures. Providing an effective and reliable seal for the piston which compresses the fluid in the cylinder can be a difficult task. A piston seal prevents any fluid from the compression chamber of the cylinder from leaking past the piston during the piston's reciprocating movement. Such piston seals are subjected to substantive wear due to the piston's movement within the cylinder bore and sometimes they fail due to excessive wear and/or due to the stress caused by the pressure exerted on the seal by the fluid being compressed. Such operating conditions can be even more challenging for high pressure reciprocating pumps and/or for pumps handling cryogenic fluids.
An example of such a reciprocating pump is a pump used to deliver gaseous fuel in liquid form from a cryogenic vessel to a gaseous fuelled internal combustion engine. Such pumps are being designed to handle fluids at relatively high pressures, for example at least 4600 psi, and at low temperatures of, for example, −130 degrees Celsius or lower. Providing an effective seal for the piston of such a pump has been found to be a difficult challenge. The differences in thermal contraction coefficients of the materials from which the pump components and the seals are made can result in gaps being formed between the piston seals and the cylinder wall allowing fluid to leak from the compression chamber past the piston seals to the low pressure side of the pump.
For hydraulic devices employing a reciprocating piston in general, seal wear due to the reciprocating movement of the piston is an inherent problem. In the past, split seals have been used to address the seal wear problem. Such split seals have the shape of a ring having a cut which allows it to be installed on the piston and more importantly it allows the seal to expand to compensate for the seal wear.
Split seals, having an S-shaped cut, are known in the industry. U.S. Pat. No. 6,305,265 describes a pump apparatus comprising a piston seal assembly illustrated in FIG. 5, with each seal member having an S-shaped cut which allows it to expand slightly in the radial direction without substantially reducing the thickness of the seal.
The disadvantage of the seals that have an S-shaped cut is that the overlapping end segments of the seal can break more easily under the stress exerted by the pressure of the fluid acting on the seal from the compression chamber.
Accordingly, there is a need for a seal design that allows the seal to expand to compensate for the seal wear and at the same time provide a robust construction of the seal which reduces the risk of seal breakage under the stress exerted by the fluid pressure in the compression chamber.